A new type of data bus was developed in the 1980's by The Boeing Commercial Airplane Company for use in connection with onboard aircraft subsystems or terminals (hereafter "terminals" and "subsystems" will be used interchangeably). This data bus provides digital autonomous terminal access communication (DATAC), and its protocol standards are now identified in the field as ARINC characteristic 629.
DATAC is designed to replace the ARINC 429 data bus system which is currently in use on some commercial aircraft. The most significant difference between the DATAC bus and the ARINC 429 bus is that DATAC is a multi-transmitter, bidirectional bus, whereas ARINC 429 is a single-transmitter, unidirectional (one-way) type bus.
The DATAC bus is similar to the Mil-Std. 1553 bus currently used on some military aircraft. The most significant difference between DATAC and Mil-Std. 1553 is that DATAC does not require a discrete data bus controller.
The problem with a discrete controller is that in the event the controller malfunctions, the entire data bus will be taken out of service. As a practical matter, this result is avoided in the Mil-Std. 1553 system by providing multiple controllers that are redundant. If one controller fails, another takes over.
The DATAC bus does not require a controller because subsystems that communicate on the bus have built-in intelligence which allows them to independently listen to the bus, and to transmit data when certain protocol conditions are met. For example, to receive data, each terminal or subsystem passively monitors the data bus. Data required by the terminal is validated and passed into the system's addressable memory. To transmit, the terminal maintains a transmission time slot on the bus. Prior to the actual transmission, the terminal first assembles a complete data message, determines when it may transmit, and then sends the message.
An excellent explanation of the DATAC bus and its protocol features may be found in U.S. Pat. No. 4,471,481, issued to John L. Shaw and Hans K. Herzog on Sep. 11, 1984. The type of aircraft subsystem that might operate on the bus would typically be autopilot, air conditioning, fire protection, control surfaces, and similar subsystems.
The DATAC bus is inherently more reliable than both the ARINC 429 and Mil-Std. 1553 busses as it does not require a single controller to operate the bus, and it has a simpler wiring configuration. One or more discrete controllers, plus wiring from the controllers to data bus subsystems, represent a significant weight and installation penalty. Reduction in weight and simplification in physical design, which DATAC provides, is always desirable in aircraft because it results in paybacks in the form of increased payload capacity and/or improved fuel economy.
Many data bus testers are microprocessor based systems whose software artificially simulates the bus to be tested. These types of testers are costly because of their software requirements, and have reliability problems in that they are sensitive to continuing software bugs for a period of time after they have been put into service. The tester disclosed herein solves this problem with respect to the DATAC bus in that it is a non-microprocessor based device which actually incorporates the DATAC VLSI chip. In other words, it has no software requirements.
U.S. Pat. No. 4,383,312 discloses a non-microprocessor based system that is used to test data buses. However, there are several notable differences between the '312 patent and the invention which is disclosed here. Although the '312 tester provides a simulated source and sink for data, a function which the present invention also accomplishes, the '312 tester simulates only a single terminal. Contrastingly, the present invention has the capability of simulating multiple terminals. Further, the '312 patent does not include the nonvolatile memory features of the present invention, and does not have the same kind of user-friendly data input/outputs. These differences, and others, will become apparent upon considering the following description.